The operation of a nuclear reactor or nuclear facility results in the creation of various radioactive elements. An important requirement of safe operation of a nuclear reactor is proper control of these elements. The control of radioactive isotopes present in the effluent gas stream from a nuclear reactor is normally accomplished by first filtering the effluent cooling gas to remove particulate materials. This leaves as the major radioactive elements of concern radioiodine and gaseous compounds thereof and radioactive noble gases. In normal operation, the quantities of either radioactive iodine or radioactive noble gases in the cooling effluent are maintained at a level low enough to present no hazard to the public health. It is possible that malfunctions in the operation of a nuclear facility may result in temporary generation of increased quantities of radioactive elements in the effluent cooling gas. It is therefore necessary to monitor the concentrations of radioactive materials in the cooling effluent. Radioactive iodine, in particular, is taken up preferentially by the human thyroid and is therefore capable of being concentrated in the body. For this reason, it is particularly important to monitor the effluent level of radioactive iodine.
It is customary to place an adsorbent filter of a material such as activated charcoal in the effluent gas from a reactor or other nuclear facility to remove radioactive iodine from the effluent gas. The filtered effluent is then monitored routinely to provide an assurance of proper operation and to provide an alert to any developing discharge of radioactive iodine resulting from failures in operation such as rupture of a fuel rod or failures in the filtering system that is designed to remove radioactive iodine from the effluent gas stream. Such monitoring efforts have involved the placement of an adsorbent filter in the effluent gas stream or in a stream sampled from the effluent gas. One means of monitoring used in the past comprised placing a detector of radiation in a position to respond to the radiation count from iodine trapped in the adsorbent filter. Another means of monitoring was disclosed in U.S. Pat. No. 3,731,100, incorporated herein by reference, in which the concentration of a radioactive substance was measured by counting total radiation in the effluent, filtering out the radioactive substance to be measured, counting total radiation in the filtered effluent, and combining the results to determine the amount of material adsorbed by filtering. The method disclosed in this U.S. Patent overcomes some disadvantages of the prior art, namely, that a high release rate of radioactive iodine followed by a low release rate gave misleadingly high readings until the residual radioactive iodine decayed to a value lower than that being deposited by the current release rate.
A remaining problem in monitoring for the presence of radioactive iodine is the fact that the effluent gas stream in a properly functioning nuclear facility has already been passed through an adsorbent filter to remove radioactive iodine. The result is the presence of a relatively small concentration of radioactive iodine in the presence of a relatively large concentration of radioactive noble gases. A typical concentration ratio of noble gas to iodine is 10,000-100,000. This has a tendency to mask variations in the concentration of radioactive iodine.
It is an object of the present invention to provide a better method and means of monitoring the concentration of radioactive iodine in the effluent gas stream from a nuclear reactor or other nuclear facility.
It is another object of the present invention to provide a method and means of detecting the concentration of radioactive iodine in the effluent stream from a nuclear facility in the presence of rapid changes of the concentration of radioactive iodine.
It is another object of the present invention to provide a method and means of detecting relatively small quantities of radioactive iodine in a stream of gas also containing radioactive noble gases.
Other objects will become apparent in the course of a detailed description of the invention.